A conventional photovoltaic cell incorporates a semiconductor structure with a junction, such as a p-n junction formed with an n-type semiconductor and a p-type semiconductor. More specifically, Si solar cells are typically made by adding controlled impurities (called dopants) to purified Si. Different dopants result in either p-type or n-type material, in which there are respectively positive or negative majority charge carriers. The cell structure includes a boundary or junction between p-type and n-type Si. When the cell is illuminated by radiation of an appropriate wavelength, such as sunlight, a potential (voltage) difference across the junction creates free charge carriers. These electron-hole pair charge carriers migrate in the electric field generated by the p-n junction and are collected by electrodes on respective surfaces of the semiconductor. The cell is thus adapted to supply electric current to an electrical load connected to the electrodes, thereby providing electrical energy converted from the incoming solar energy that can do useful work. For the typical p-base configuration, a negative electrode is located on the side of the cell that is to be exposed to a light source (the “front” side, which in the case of a solar cell is the side exposed to sunlight), and a positive electrode is located on the other side of the cell (the “back” side). Solar-powered photovoltaic systems are considered to be environmentally beneficial in that they reduce the need for fossil fuels used in conventional electric power plants.
Industrial photovoltaic cells are commonly provided in the form of a structure, such as one based on a doped crystalline silicon wafer, that has been metalized, i.e., provided with electrodes in the form of electrically conductive metal contacts through which the generated current can flow to an external electrical circuit load. Most commonly, these electrodes are provided on opposite sides of a generally planar cell structure. Conventionally, they are produced by applying suitable conductive metal pastes to the respective surfaces of the semiconductor body and thereafter firing the pastes.
Photovoltaic cells are commonly fabricated with an insulating layer on their front side to afford an anti-reflective property that maximizes the utilization of incident light. However, in this configuration, the insulating layer normally must be removed to allow an overlaid front-side electrode to make contact with the underlying semiconductor surface. The front-side conductive metal paste typically includes a glass frit and a conductive species (e.g., silver particles) carried in an organic medium that functions as a vehicle for printing. The electrode may be formed by depositing the paste composition in a suitable pattern (for instance, by screen printing) and thereafter firing the paste composition and substrate to dissolve or otherwise penetrate the insulating anti-reflective layer and sinter the metal powder, such that an electrical connection with the semiconductor structure is formed.
The ability of the paste composition to penetrate or etch through the anti-reflective layer and form a strong adhesive bond with the substrate upon firing is highly dependent on the composition of the conductive paste and the firing conditions. Key measures of photovoltaic cell electrical performance, such as efficiency, are also influenced by the quality of the electrical contact made between the fired conductive paste and the substrate.
Although various methods and compositions useful in forming devices such as photovoltaic cells are known, there nevertheless remains a need for compositions that permit fabrication of patterned conductive structures that provide improved overall device electrical performance and that facilitate the efficient manufacture of such devices.
One common class of Si solar cell designs employs a 200 μm thick p-type Si wafer with a 0.4 μm layer of n-type Si on the wafer's front surface. The p-type wafer provides the base and the n-type layer is the emitter. In various implementations the n-type layer is made by either diffusing or ion implanting phosphorus (P) dopant into the Si wafer.
Dopant concentration must be controlled to achieve optimal cell performance. A high dopant concentration in the emitter imparts low electrical emitter sheet resistivity and enables a low resistivity metal contact to be made at the Si surface, thereby decreasing resistance losses. However, a high dopant concentration also introduces crystalline defects or electrical perturbations in the Si lattice that increase recombination losses that reduce both the current and voltage of the cell.
As known to one skilled in the art (see, e.g., S. W. Jones, “Diffusion in Silicon,” IC Knowledge LLC (2008), pp. 56-62), total dopant concentration is typically measured using a SIMS (secondary ion mass spectrometry) depth profiling method and active dopant concentration is measured using SRP (spreading resistance probing) or ECV (electrochemical capacitance voltage) methods.
The wafers most commonly used in conventional photovoltaic cells prepared with emitters that have a total concentration of P dopant at the surface [Psurface] ranging from 9 to 15×1020 atoms/cm3. The active [Psurface] typically ranges from 3 to 4×1020 atoms/cm3. Such emitters are termed as highly or heavily doped emitters (HDE), and the wafers incorporating those emitters are often referenced simply as “HDE wafers.”
A concentration of P dopant at the front surface ([Psurface]) above ˜1×1020 atoms/cm3 in Si leads to various types of recombination. Recombined charge carriers are bound to the Si lattice and unable to be collected as electrical energy. The solar cell energy loss results from a decrease of both Voc (open circuit voltage) and Isc (short circuit current). P dopant in excess of the active concentration (inactive P) leads to Shockley-Read-Hall (SRH) recombination energy loss. Active P dopant above ˜1×1020 atoms/cm3 leads to Auger recombination energy loss.
Emitters made with low dopant concentration at the wafer surface are called lightly or low-doped emitters (LDE). A wafer having [Psurface]<1×1020 atoms/cm3 is typically termed an LDE wafer. Solar cell embodiments employing lightly doped emitters in some instances achieve improved solar cell performance by decreasing the losses resulting from electron-hole recombination at the front surface. However, the inherent potential of LDE-based cells to provide improved cell performance is frequently mitigated in practice by the greater difficulty of forming the high-quality metal contacts needed to efficiently extract current from the operating cell.
As a result, wafers used for commercial solar cells typically employ high [Psurface] emitters, as discussed above, which degrade short wavelength response (short wavelengths have a very high absorption coefficient in silicon and are absorbed very close to the surface) and result in lower open-circuit voltage (Voc) and short-circuit current density (Jsc). The high [Psurface] emitters enable formation of low contact resistivity metallization contacts, without which contact is poor and cell performance is degraded.
Nevertheless, an improvement in cell performance is potentially attainable with LDE-based cells. Such cells would require a thick-film metallization paste that can reliably contact lightly doped, low [Psurface] emitters without damaging the emitter layer surface, while still providing low contact resistance. Ideally, such a paste would enable screen-printed crystalline silicon solar cells to have reduced saturation current density at the front surface (J0e) and accompanying increased Voc and Jsc, and therefore improved solar cell performance. Other desirable characteristics of a paste would include high bulk conductivity and the ability to form narrow, high-aspect-ratio contact lines in a metallization pattern to further reduce series resistance and minimize shading of incident light by the electrodes, as well as good adherence to the substrate.